1. Field of the Invention
The present invention relates to a computed tomography (CT) device for generating tomograms of slices of an examination subject, of the type wherein the slices are inclined toward the longitudinal axis of a patient support.
2. Description of the Prior Art
In a CT device, it is necessary to be able to tilt the system axis and the longitudinal axis of the patient support relative to one another in order to be able, in the tilted state, to display objects whose center axis is inclined toward the center axis of the patient support, such as discs.
In conventional CT devices, with a detector system having a single row of detector elements, it is possible also to execute spiral scans in a tilted state, wherein the X-ray source continuously rotates around the system axis and the patient support is translationally adjusted in the direction of its longitudinal axis.
U.S. Pat. No. 5,574,763 discloses a CT device for spiral scanning of patients in an upright posture, whereby, instead of a movement of the patient support in the direction of its longitudinal axis, a relative movement of gantry and patient support is generated by vertically moving the gantry and by horizontally moving the patient support, so that the relative movement proceeds in the direction of the longitudinal axis of the patient support.
U.S. Pat. No. 4,989,142 discloses a method for acquiring data which are necessary for displaying three-dimensional surfaces, from tomograms of slices that are inclined toward the system axis, wherein these tomograms are picked up by a conventional CT device.
Spiral scans in a tilted state are problematic with respect to newer CT devices having a detector system with a number of rows of detector elements. These problems are based on arise because methods which are normally used in the course of the image reconstruction for the spiral interpolation, are not utilizable for the tilted state of the CT device and, moreover, complicated methods for the spiral interpolation are necessary, which serve the purpose of calculating an image of a single slice from data acquired during a spiral scan. Problems also arise because conventional methods are not utilizable for eliminating ring artefacts in the tomograms acquired in the tilted state.
An object of the present invention is to provide a CT device of the type initially described wherein conventional methods can be utilized for the spiral interpolation and for eliminating ring artefacts, even though a detector system having a number of rows of detector elements is used.
According to the invention, this object is achieved in a computed tomography (CT) device for generating tomograms of slices of the examination subject, the slices being inclined toward the longitudinal axis of a patient support for an examination subject, wherein the CT device has an X-ray source, which can be displaced around a system axis, and a detector system with a number of rows of detector elements, the X-ray source and the detector system being mounted at a gantry, wherein the gantry and the patient support can be tilted relative to one another such that the system axis and the longitudinal axis of the patient support are inclined relative to one another in the tilted state, and wherein, in the tilted state, the patient support and the gantry can be substantially linearly adjusted relative to one another in the direction of the system axis for generating a relative movement between the gantry and the patient support.
In contrast to conventional devices, the relative movement between the gantry and the patient support does not ensue in the direction of the longitudinal axis of the patient support but, instead ensues in the direction of the system axis. It is thus possible to utilize known methods for the spiral interpolation and for eliminating ring artefacts, although the CT device is operated in the tilted condition.
In a version of the invention the system axis extends substantially horizontally in the tilted state. In this case, tilting of the gantry can be foregone, since the longitudinal axis of the patient support assumes a direction that is inclined relative to the horizontal, however, such a position of the patient support can be somewhat uncomfortable for patients.
In another version of the invention the longitudinal axis of the patient support extends substantially horizontal in the tilted state. In this case, an inclination of the patient support that may be unpleasant for the patients is avoided, however, appropriate actions must be taken, so that a relative movement between the patient support and the gantry can be executed in the direction of the system axis, given a horizontally oriented longitudinal axis of the patient support.
While only one drive is necessary in the case of the horizontal arrangement of the system axis in order to effect the relative movement between the inclined patient support and the gantry in the direction of the system axis, two drives can be necessary in the case of the horizontal orientation of the longitudinal axis of the patient support. These drives make it possible, for example, to adjust the patient support in horizontal direction and the vertical direction in a synchronized fashion such that a relative movement between the gantry and patient support results in the direction of the system axis. In order to avoid the use of two drives, the patient support with a horizontally oriented support plate can be adjusted on bars that are tilted toward the horizontal to achieve a desired inclination. The support plate can be horizontally oriented by means of an adjustable piston or the like, even when different inclinations of the bars are possible.
In a preferred embodiment of the invention, in the tilted state, the patient support can be adjusted relative to the stationary gantry in the direction of the system axis for generating the relative movement between the gantry and the patient support, since an adjustment of the patient support is normally easier to carry out than an adjustment of the gantry. Adjustment of the gantry or a common adjustment of gantry and patient supports are still possible, however, in order to achieve the relative movement in the direction of the system axis.